I love canoeing; but I'm also a rag man. I want the option of sails on my canoe. I know it can be done elsewhere, but what about in the boundary waters? "No mechanical devices," yes; but what constitutes a mechanical device? Sheet winches, travellers, and vangs - probably. But what about a rudder (as on some kayaks)? Is an outrigger temporarily fastened to a canoe a "mechanical device"? Or a fixed mast? If the bow man folds a bed sheet over a paddle and holds it up while the stern man grasps the free corners, would that be considered using a "mechanical device"? (It might be a good way to get wet.) Don't get me wrong. Paddling across a lake has its own challenges and rewards; and purists might scoff at the idea of using a sail. I can appreciate that, and I wouldn't want the boundary waters filled with a host of sail powered canoes speeding across the lakes at 10plus knots. But sailing is just as environmentally friendly as paddling. I don't want to break the rules, just understand them.
On a trip up Agnes in Quetico we tried a sail. We found two trees and stripped the dead branches from them. We used a large tarp rolled the tarp around them with some lashing and form a triangle. It worked OK but we did not spend the time to fix it to the canoe well. I think sailing would be great it you can get some basic equipment and it was "legal". I think the big issue is getting the sail to attach to the canoe to transfer the power.
Wind at your back is always a great way to travel in the bwca area. We use a light weight tarp, that also doubles as our dining fly, when we have favorable winds. We simply hold the fly high enough in front with our paddles to catch the wind. We recently flew across Lake Agnes without any paddling and were across the whole length in minutes. I'm not sure if this is with-in the "rules" of the BWCA but the few times that we have done this the experience has been great. Our group was able to lash the canoes together and "enjoy" some free time and yet keep traveling with little effort.
Here's another twist for thought. I've been pondering this for years, since playing with sport kites. Parafoil kites in the two-line version can be easily steered within maybe a 150 degree window (or so). They come in a wide variety of sizes (different powers) and do not attach to the boat in any way. I'm not familiar with the fine details of the law, would kites be allowed?
Also an easy way to rig a sail when the wind is with you is to use a poncho. Bowman sticks the paddle through to the hood, and holds the handle between his legs, he then holds the corners of the poncho out.